This invention relates generally to building construction, and more particularly to a mid-roof anchoring system for large metal roofs.
When designing a metal roof, one has to allow for thermal expansion, since roof temperature can vary substantially, during the course of a year, from the coldest annual temperature for the locale to a temperature well above (because of radiant heating by the sun) the highest annual temperature. Linear growth of a particular roof span is proportional to span length, so expansion problems become more acute as roof size increases.
For a metal roof of modest size, the roof covering may be affixed to the substructure along one edge thereof, for example along the eave, and allowed to shrink or grow elsewhere. The roof may be secured to the substructure, other than at the fixed edge, by clips which permit sliding movement between the covering and the substructure. Butler Manufacturing's MR-24 clips, for example, permit two and one-half inches of movement, i.e., one and a quarter inches either way from a neutral position. The upper edges of the roof move with respect to the roof ridge line as the roof expands and contracts. The ridge is covered by a ridge cap, which may comprise a U-shaped element which can bend to accommodate roof expansion. Flexible weather seals may be provided at the interface.
For large roof spans (that is, continuous panel runs not interrupted by thermal expansion joints), on the order of 200 to 300 feet, depending on the geographic location, movement of the free edge of the roof may exceed the design limits of the attachment clips. One way to overcome this problem is to break the roof span into two separate spans having a step or lap joint, like very large shingles. The uppermost span is secured along the step, and expands toward the roof ridge line, and the lowermost span is affixed along the eave. Where the spans overlap, the lower span slides or "floats" beneath the other.
A problem with stepped roofs is that of weather sealing, particularly leak prevention, at the lap joints, While excellent weather seals exist, it would be simpler, cheaper and better to be able to provide a large roof with long continuous spans, so that steps were not required.
As shown in FIG. 1, a typical metal building includes an array of vertical members 10, interconnected by substantially horizontal beams 12, and supporting a roof substructure 14. The roof substructure includes a series of parallel main frames or trusses 16, or their functional equivalent, each running from the roof ridge 18 to an eave 20. The main frames, in turn, support parallel purlins 22, or their equivalent, each running parallel to the ridge line and eaves. The main frames and purlins may be continuous or segmented, probably the latter for the large roofs.
The purlins are covered by metal panels 24, which are seamed edge-to-edge, by rolling their edges 26 together. The panels are conventionally held to the roof by clips 30 (see FIG. 5) which permit some lengthwise movement of the panels as they expand and contract with respect to the substructure. The clips 30 may be of the type shown in U.S. Pat. No. 4,543,760, which is incorporated by reference. Each of these clips has a sliding element with sheet metal tabs 28 which are rolled into the roof seam as it is formed.
The edges of the panels are raised substantially, FIG. 4, so that the completed roof is in a sense corrugated. Reference may be made to U.S. Pat. No. 4,559,753 for a more thorough description of the panels, and to U.S. Pat. No. 4,989,308 for a description of an apparatus for forming the seams in situ. Both patents are incorporated herein by reference.
Optionally, a layer of insulation 32 may be laid over the purlins, before the roof panels are installed.
If such a construction is used for very large buildings, roof expansion may produce movement exceeding the design limits of the attachment clips; a stepped or overlapped assembly of separate panel spans (see FIG. 2) is then ordinarily required, but such an expedient is objectionable from several standpoints, including the cost of additional parts, and problems with long term leak prevention, snow catching and vapor retarder integrity.